There are wide varieties of molecular sieves for use in commercial petroleum and petrochemical industry processes. Molecular sieves are porous solids having pores of varying sizes. The most commercially useful molecular sieves are known as zeolites. Zeolites in general have a one-, two- or three-dimensional crystalline pore structure having uniformly sized pores of molecular dimensions that selectively adsorb molecules that can enter the pores, and exclude those molecules that are too large.
Some examples of molecular sieves are aluminosilicates and metalloaluminophosphates, such as silicoaluminophosphates (SAPOs). SAPO molecular sieves contain a three-dimensional microporous crystalline framework structure of [SiO4], [AlO4] and [PO4] corner-sharing tetrahedral units. SAPO synthesis is described in U.S. Pat. No. 4,440,871, which is herein fully incorporated by reference. SAPO molecular sieves are generally synthesized by the hydrothermal crystallization of a reaction mixture of silicon-, aluminum- and phosphorus-sources and at least one templating agent. Synthesis of a SAPO molecular sieve, its formation into a catalyst, and its use in a process for converting hydrocarbon feedstock into olefin(s), particularly where the feedstock is methanol, are disclosed in a large number of granted patents and pending patent applications, including for example, U.S. Pat. Nos. 4,499,327, 4,677,242, 4,677,243, 4,873,390, 5,095,163, 5,714,662 and 6,166,282 and U.S. Patent Application Publication Nos. 20050096214 and 20050101818 (all incorporated herein by reference).
Typically, molecular sieves are formed into molecular sieve catalyst compositions to improve their durability in commercial processes. These molecular sieve catalyst compositions are formed by combining the molecular sieve and a matrix material usually in the presence of a binder. The purpose of the binder is to hold the matrix material, often clay, to the molecular sieve. Binders and matrix materials typically only serve to provide desired physical characteristics to the catalyst composition, and have little to no effect on conversion and selectivity of the molecular sieve.
Olefins are traditionally produced from petroleum feedstocks by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s), such as ethylene and/or propylene, from a variety of hydrocarbon feedstocks. Ethylene and propylene are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds.
Alternately, the methanol-to-olefins (MTO) process can be used to produce light olefins by contacting a feedstock containing oxygenated organic compounds, such as methanol or dimethyl ether (DME), with a molecular sieve catalyst, such as a SAPO molecular sieve. Methanol, the preferred alcohol for light olefin production, is typically synthesized from the catalytic reaction of hydrogen, carbon monoxide and/or carbon dioxide in a methanol reactor in the presence of a heterogeneous catalyst. For example, in one synthesis process methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular methanol reactor.
Many attempts have been made to improve a molecular sieve catalyst composition's selectivity and/or extend its lifetime by modifying the catalyst composition. For example, U.S. Pat. No. 6,180,828 discusses the use of a modified molecular sieve to produce methylamines from methanol and ammonia where, for example, a silicoaluminophosphate molecular sieve is combined with one or more modifiers, such as zirconium oxide, a titanium oxide, yttrium oxide, montmorillonite or kaolinite.
U.S. Pat. No. 4,465,889 describes a catalyst composition comprising a silicate molecular sieve impregnated with thorium, zirconium, or titanium metal oxide for use in converting methanol, dimethyl ether, or a mixture thereof into a hydrocarbon product rich in iso-C4 compounds.
U.S. Pat. No. 6,906,232 relates to a conversion process of a feedstock, preferably an oxygenated feedstock, into one or more olefin(s), preferably ethylene and/or propylene, in the presence of a molecular sieve catalyst composition that includes a molecular sieve and a Group 3 metal oxide and/or an oxide of a Lanthanide or Actinide series element. The invention is also directed to methods of making and formulating the molecular sieve catalyst composition useful in a conversion process of a feedstock into one or more olefin(s).
Other patents and publications of relevance to this invention include:    U.S. Pat. No. 6,906,232, U.S. Pat. No. 6,844,291, U.S. Pat. No. 6,951,830,    U.S. Pat. No. 4,302,622, U.S. Pat. No. 4,590,323, U.S. Pat. No. 4,929,763,    U.S. Pat. No. 5,043,308, U.S. Pat. No. 5,130,114, U.S. Pat. No. 5,189,198,    U.S. Pat. No. 6,017,442, U.S. Pat. No. 6,287,527, U.S. Pat. No. 6,600,056,    EP 1478464 U.S. Patent Pub. No. 20030171633, U.S. Patent Pub. No. 20030181325, U.S. Patent Pub. No. 20050054517 and U.S. Patent Pub. No. 20050020435.
Molecular sieve catalysts, including SAPO molecular sieve catalysts, when used to convert methanol to olefins, require frequent regeneration due to coking and therefore have limited lifetimes. Coking occurs when coke deposits either directly (site coverage) and/or indirectly (pore blockage) decrease the number of active sites available for the conversion reaction in the reactor. Coking is a common cause of catalyst deactivation, thereby decreasing catalyst lifetime.
Regeneration is the process whereby at least a portion of the molecular sieve's initial activity is recovered by combusting and removing at least a portion of the coke deposits on the catalyst with agents such as air, hydrogen, steam, or carbon monoxide, alone or in combination. This process is very expensive, time consuming and adds extra steps to the conversion process. It would be beneficial to reduce coking, thereby increasing catalyst lifetime and reducing the need for regeneration, which would decrease the cost of the entire conversion process. Therefore, any means for reducing coke selectivity or increasing catalyst lifetime would result in significant investment savings.
It would also be desirable to have an improved molecular sieve catalyst composition for use in MTO processes having a better conversion rate, improved olefin selectivity and a longer lifetime.
The present invention satisfies these needs by providing a molecular sieve catalyst composition and a method for preparing the molecular sieve catalyst composition in which a metal carbonate is mixed with the molecular sieve followed by calcination of the mixture. In a preferred embodiment, the catalyst composition is prepared by mixing a metal carbonate with a molecular sieve catalyst, for example SAPO-34, followed by calcination. This results in significantly longer catalyst lifetime when compared with a molecular sieve catalyst composition with no metal carbonate. Also the calcination temperature used has a profound impact on coke selectivity, and catalyst lifetime.